Heaters comprised of an inner resistance or heating coil surrounded by an insulating material that is in turn surrounded by an outer metal sheath are well known in the art. Such heaters are resistance heaters because with the passing of a current through the resistance coil, the resistance coil begins to warm up and produce heat. The heat is dissipated through the insulating material and the sheath to provide external heat. The amount of heat produced by the heater is dependent upon many factors, some of which are the size of the wire used for the resistance coil, the type of wire, the spacing of the coils of the resistance coil, and the amount of current allowed to pass through the resistance coil. The amount of heat produced per unit area of the heated portion of the heater is known as the watt density of the heater. The watt density is a function of the applied voltage and the electrical resistance per a linear unit of length of the resistance wire coil. The electrical resistance per linear unit of length, generally an inch, may be termed the specific resistance of the heater. Therefore, a given length of the resistance coil will have a given or specific electrical resistance.
In many applications, it is necessary to provide a heater that has a heatable or hot heater section disposed between two non-heatable or cold heater sections. The cold heater section is generally a terminal pin or power pin surrounded by an insulating material that is in turn surrounded by a metal sheath. The power pin is coupled to the resistance coil of the hot heater section. Voltage is applied to the power pin which conducts the current to the resistance coil to generate heat. In other applications it is necessary to manufacture heaters with varying watt density along the length of the heater. Many variations in heater assembly necessitate the custom manufacture of heaters.
With today's technology, the length of the heater's heated and cold sections are subjected to variation of manufacturing. The most common variables are the density of the insulating material, typically magnesium oxide (MgO), tube wall thickness, fill machine and compacting machine variations, and operator variation. These variables make it very difficult to fabricate heaters with consistent and tight tolerance in the heated and cold sections. Typically, the current heater uses 1-1.5% length tolerance for the heated or cold section. However, in real applications, this variation may not be desirable.
Because of the limitations of the machines used to fabricate the heaters, heaters required to be longer than the machine limitations must be spliced together. Therefore, in the prior art, heater manufacturing is custom manufacturing in almost every application.
It is thus an object of the present invention to provide a method for heater fabrication that produces consistent hot and cold heater sections to within 1/8 regardless of the design length.
It is another object of the present invention to provide a method of heater fabrication that imposes no limitations on length.
It is further an object of the present invention to provide a method of fabricating heaters wherein only stock lengths of heater sections, both hot and cold, may be premanufactured and used in assembling the required heater.
It is still further an object of the present invention to accurately fabricate multiple or variable watt density heaters.